The Tower Subway is a tunnel, dug in 1869, beneath the River Thames in central London, close to the Tower of London. Its alignment runs between Tower Hill on the north side of the river and Vine Lane (off Tooley Street) to the south. Its innovative method of construction provided the template for the construction in 1890 of the City & South London Railway, the first of London's "Tube" railways. The tunnel is called a subway according to normal British usage, indicating a tunnel for pedestrians, and not to the American English usage, equivalent to underground rapid transit. It is sometimes cited as the world's first underground tube railway, though it was not the first underground railway.

Origins of the Tower Subway The construction of the Tower Subway was authorised by an Act of Parliament in 1868, which established the Tower Subway Company with three Directors and a capital of £12,000. However, at first no contractor could be found willing to undertake the work, due to the immense difficulties and expense incurred by Marc Isambard Brunel's Thames Tunnel. However, James Henry Greathead, then only 24, tendered for the construction of the tunnel and access shafts for the sum of £9,400, with Peter W. Barlow and Barlow's son Peter W. Barlow Jr. as resident engineer and designed and built the tunnel in 1869–1870. In conjunction with Barlow Sr. he designed a cylindrical wrought iron tunnelling shield, the Barlow-Greathead shield, which greatly improved on Marc Isambard Brunel's shield developed for the construction of the Thames Tunnel. Greathead's shield was effectively a wrought iron sleeve with the same diameter - 7 feet 3 inches (2.21 m) - as the tunnel. As excavation progressed, it was forced ahead by hand-operated screw jacks to act both as a ring‑shaped cutter and protection for the workmen. It was 4 feet 9 inches (1.45 m) long, 0.5 inches (13 mm) thick, and weighed 2.125 long tons (2.159 t). Unlike Brunel's large and unwieldy rectangular shield, which weighed 120 tons, it moved in one piece. The shield was made with a slight taper at the front to reduce the friction of the surrounding clay, and the front of the cylinder was stiffened by a cast‑iron ring bolted to it; behind which was a diaphragm, or bulkhead, with a hatchway through which the workers could pass to the face. Work on the Tower Subway began in February 1869, with the boring of entrance shafts on each side of the river at slightly different depths. The north shaft at Lower Thames Street on Tower Hill is 60 feet (18 m) deep, while that on the south bank at Vine Lane is 50 feet (15 m) deep. The two shafts were later fitted with steam-powered lifts for passenger use. The tunneling itself started in April and finished in December and was bored through a fortuitously stable layer of the London Clay that lay 22 feet (6.7 m) below the river bed, and below the soft alluvial deposits that had so plagued the construction by Brunel of the earlier Thames Tunnel a few miles further downstream. This, combined with the much simpler nature of the project — the excavation face was only one twentieth that of the Thames Tunnel — enabled very rapid progress. The under-river section was completed in only fourteen weeks and the entire tunnel was constructed in less than a year, marking a vast improvement over the tortuous progress of the Thames Tunnel. It also marked a significant technical advance — for the first time, cast iron rather than brick was used to line the tunnel, segments being put into place behind the tunneling shield as it moved forward so that the shield's screw jacks could push against it. The Subway was originally intended to provide a passenger service beneath the river. A small cable car (dubbed an omnibus by the tunnel's operators) shuttled a maximum of 12 passengers from end to end through a single bore, 450 yards (410 m) long and 7 feet (2.1 m) in diameter, on 2 feet 6 inches (760 mm) gauge track. The journey, powered by a 4 hp (3 kW) stationary steam engine on the south side of the tunnel, took about 70 seconds. However, the cramped, low-capacity Subway proved uneconomical almost as soon it was officially opened on 2 August 1870: the railway service lasted just three months, too few passengers being carried and the speed of the carriage being too slow, and the Tower Subway went into receivership in November 1870. The tunnel was subsequently converted to a pedestrian walkway with the cable car and steam engine removed, gas lights installed and the passenger lifts replaced with spiral staircases. It became a very popular way to cross the river, averaging 20,000 people a week (a million a year) at a cost of a halfpenny each way. Its main users were reportedly "the working classes who were formerly entirely dependent on the ferries". In September 1888 the Subway briefly achieved a certain notoriety after a man brandishing a knife was seen in the tunnel at the time when Jack the Ripper was committing murders in nearby Whitechapel. Tower Subway was eventually superseded by Tower Bridge, which was constructed a few hundred yards downriver and opened in 1894. This caused an immediate collapse in the Subway's income, as the bridge — unlike the tunnel — provided a toll-free crossing. There was thus no longer any incentive to use the Subway. In 1897, Parliament passed an Act authorising the sale of the tunnel to the London Hydraulic Power Company for £3,000, and the Subway finally closed to passenger traffic in 1898.

Contemporary accounts of the Subway The Subway was, by all accounts, not a good place for a claustrophobe to visit. Charles Dickens, Jr reported that "there is not much head-room left, and it is not advisable for any but the very briefest of Her Majesty's lieges to attempt the passage in high-heeled boots, or with a hat to which he attaches any particular value." The Italian writer Edmondo De Amicis (1846–1908) gave a vivid description of a passage through the Subway in his Jottings about London: "As I was thinking of these things I disappeared from the world indeed, going down a lighted spiral staircase which buries itself in the earth on the right bank of the Thames, opposite the Tower. I went down and down between two dingy walls until I found myself at the round opening of the gigantic iron tube, which seems to undulate like a great intestine in the enormous belly of the river. The inside of this tube presents the appearance of a subterranean corridor, of which the end is invisible. It is lighted by a row of lights as far as you can see, which shed a veiled light, like sepulchral lamps; the atmosphere is foggy; you go along considerable stretches without meeting a soul; the walls sweat like those of an aqueduct; the floor moves under your feet like the deck of a vessel; the steps and voices of the people coming the other way give forth a cavernous sound, and are heard before you see the people, and they at a distance seem like great shadows; there is, in short, a sort of something mysterious, which without alarming causes in your heart a vague sense of disquiet. When then you have reached the middle and no longer see the end in either direction, and feel the silence of a catacomb, and know not how much farther you must go, and reflect that in the water beneath, in the obscure depths of the river, is where suicides meet death, and that over your head vessels are passing, and that if a crack should open in the wall you would not even have the time to recommend your soul to God, in that moment how lovely seems the sun! I believe I had come a good part of a mile when I reached the opposite opening on the left bank of the Thames; I went up a staircase, the mate of the other, and came out in front of the Tower of London."

The Subway today After its closure, the tunnel gained a new purpose as a route for hydraulic tubes operated by the London Hydraulic Power Company, and water mains. It was badly damaged by bombing during World War II, when a German bomb fell in the river near Tower Pier in December 1940 and exploded on the river bed very close to the tunnel's roof. The shock of the blast compressed the tunnel radially, reducing its diameter to only 4 feet (1.2 m) at the point of impact, but fortunately the tunnel's lining was not penetrated. During the course of repair work, it was found that — apart from the bomb damage — the tunnel had survived seventy years of use in excellent condition. While it is no longer used for hydraulic tubes, the tunnel still carries water mains. The hydraulic tubes, once a major source of power in the centre of London, have since been replaced by telecommunication cables. A small round entrance building survives at Tower Hill near the Tower of London's ticket office, a short distance to the west of the main entrance to the Tower. This is not the original entrance, but was built in the 1920s by the London Hydraulic Power Company. A ring of lettering gives the date of construction and names the LHPC, as if it had built the subway originally, which it did not. The corresponding entrance on the south bank of the Thames was reportedly demolished in the 1990s.